An organic light emitting device (OLED) includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron-transporting layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full-color flat emission displays. Tang et al. described this multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
Physical vapor deposition in a vacuum environment is the principal means of depositing thin organic material films as used in small molecule OLED devices. Such methods are well known, for example Barr in U.S. Pat. No. 2,447,789 and Tanabe et al. in EP 0 982 411. Linear deposition sources of the prior art typically are capable of achieving thickness uniformity specifications of ±10% and have more recently achieved ±4%. This has been adequate for early OLED devices but is not adequate for OLED devices that rely on the organic layer stack thickness to act as a resonance cavity and thereby increase the intensity of the emitted light. For these devices to be effective, it is necessary to control the cavity thickness to within ±1 to 2%.
To achieve these thickness uniformity objectives, it is necessary to control the uniformity of the vapor flow through the source exit aperture or apertures and to additionally tailor the flow in a manner that compensates for losses at the edges of the substrate. The observed film thickness from a source with uniform vapor flow along its length depositing a film on a equal size substrate shows a fairly uniform central region that is bordered by end regions where the film thickness declines at an increasing rate toward the edge of the substrate.
Increasing the distance from the source to the substrate, known as the throw distance, and increasing the length of the source relative to the width of the substrate has the effect of increasing the thickness uniformity over the substrate. The prior art has achieved increased thickness uniformity by increasing the throw distance and using sources that may be as much as twice as wide as the substrate. For example, International Patent Application WO 2003/062486 discusses the need to increase throw distance as the substrate size increases. This strategy requires large deposition chambers, results in very low deposition rates on the substrate and wastes the vast majority of sublimated organic material.
To reduce the disparity between the length of the source and the width of the substrate, dog-bone shaped slit apertures and the use of discrete apertures whose size or packing density increases toward the ends of the source have been described by Lee et al. in International Patent Application WO 03/079420 A1 to deliver higher vapor flow at the ends of the substrate and thereby compensate for the usual thickness decline. This practice improves the thickness uniformity, but it has proven impossible to reduce a residual sinusoidal thickness variation at the end of the substrate to the 1% level.
For linear sources operated at short throw distances, there is less thickness variation of the deposited film between the ends of the substrate and the center, but local variations in vapor plume density become apparent in the deposited film. This is especially true in sources where the exit aperture is composed of multiple discrete orifices. Perhaps more problematic to the substrate, the organic film, and any masks used to define pixels, is the increased heat radiated from the source to the substrate at short throw distances. The increased heating alters the organic film nucleation process and dilates the substrate and mask, making it impossible to maintain precise alignment.